1. Field of the Invention
The present invention relates to a water-soluble resin-based airbag base fabric coating material, an airbag base fabric, and a method for manufacturing thereof and, in particular, to an airbag base fabric coating material that can form a coating film (coat) having excellent heat and moisture resistance (hydrolysis resistance) and flexibility on an airbag base fabric.
Hereinafter, “parts” in a blending unit will be in a mass unit unless otherwise noted.
2. Description of Related Art
An airbag device for protecting occupants is mounted in a vehicle. For the airbag base fabric incorporated in the airbag device, a cloth composed of (for example, plain-woven) fiber yarns (for example, polyamide fiber and polyester fiber) is used.
The purpose of the airbag is to protect occupants in a vehicle. As a basic performance, the airbag should swell in an instant and have an air shutoff property for securing an air pressure for a sufficient time period (see Lines 5 to 7 in Paragraph of Patent Document 1).
In order to ensure an appropriate breathability (air barrier property) on the airbag base fabric, one or both of the sides of the cloth have been coated with a coat (coating film) of silicon elastomer resin or urethane resin (see Paragraph [0003] of Patent Document 1).
In addition, all of the coating materials of such coating films have been emulsion coating materials (see Abstract of Patent Documents 2 and 3, for example).
However, since preparation of such emulsion coating materials is troublesome and many of the additives are expensive, the coating materials are likely to relatively increase in cost.
Thus, for example, a water-soluble resin-based aqueous coating material such as polyvinyl alcohol (PVA) may be used to form a coat (coating film) on one or both of the sides of the cloth (base fabric) (see Examples 1 to 4 of Patent Document 1, for example).
In this case, the airbag is required to have heat and moisture resistance to exert a coating performance under a heated and humidified condition in a parked vehicle (see Paragraph [0002] of Patent Document 1, for example).
On the other hand, cloths having a relatively low cover factor (K) tend to be used, because weight reduction, cost reduction, etc., have recently been required for the airbag base fabric.
It should be noted that the cover factor (K) is represented by the following formula (1).K=NW×DW0.5+NF×DF0.5,  (1)
where NW represents warp density (threads/in), DW represents warp fineness (denier), NF represents weft density (threads/in), and DF represents weft fineness (denier).
The cover factor (K) being low or high means that the warp and weft densities and/or the warp and weft finenesses are relatively low or high.
In the case of a cloth having a low cover factor, the coating film has been required to have higher heat and moisture resistance as well as higher flexibility (extensibility) than in the past from the viewpoint of the exertion of the above-described performance.
However, in the case of water-soluble resin, improvement in the heat and moisture resistance is basically limited, and furthermore, most are hard and brittle, having been considered to be difficult to meet the requirement above.
Hence, Patent Document 4 proposes, as follows, a water-soluble resin-based airbag base fabric coating material that can form a water-insoluble coat (coating film) having excellent heat and moisture resistance and flexibility on one or both of the sides of a cloth (see claim 1).
“A water-soluble resin-based aqueous airbag base fabric coating material, wherein the water-soluble resin is polyvinyl alcohol (PVA), and wherein aliphatic polyvalent carboxylic acid or polyisocyanate reactive with OH groups of the PVAL is added as a cross-linker and liquid polyol is added as a plasticizer.”
In addition, the recent environmental changes (global warming and humidification) have been requiring higher heat and moisture resistance than ever before. For example, in a hot water immersion test for heat and moisture resistance evaluation, improvement in the leaching rate has been required, but it has been difficult for aliphatic polyvalent carboxylic acid for PVA cross-linkage to meet the requirement. Since the bond between PVA and aliphatic polyvalent carboxylic acid is an ester bond, there has been a worry that hydrolysis due to humidification and heating could damage the coating film.
To address this, polyisocyanate such as MDI or HMDI (HDI) that can generate urethane bonds with PVA may be used for PVA cross-linkage (see FIG. 1 of Patent Document 4). However, with polyisocyanate such as MDI or HMDI, it has been difficult for the coating film to have sufficient flexibility, that is, to ensure a sufficient breaking extension (EB). In addition, MDI and HMDI are also reactive (can form a bridge) with water, suffering from a stability problem as a coating material. It should be noted that MDI and HMDI in Patent Document 4 are monomers, both included in a low-molecular-weight form in the present invention.
Further, Patent Document 5 discloses a water-dispersible polyurethane coating material, in which blocked polyisocyanate (B-NCO) is added as an adhesion (adherence) improver to a base fabric. It is also described that the B-NCO may be a medium-molecular-weight form as a polyether-modified prepolymer used in the present invention or a low-molecular-weight form (monomer) (see Paragraphs [0026] to [0028]). However, Patent Document 5 is directed to a water-dispersible polyurethane coating material, the blending ratio of which to the base resin is 0.1 to 3 mass % (see Paragraph [0025]), which is largely different from the blending ratio of 10 to 30 parts to 100 parts of PVA in the examples of the present invention. Further, this document includes no disclosure or indication regarding addition of a low-molecular-weight form in a small amount to the medium-molecular-weight form of the B-NCO for improvement in the adhesion to the base fabric.
Furthermore, to achieve an improvement in the adherence of a cross-linked coat (breathability reducing coating film) to a cloth, Patent Document 6 discloses a coating film configuration using a coating material such as silicon resin (considered to be emulsion-based or solution-based) to partially enclose each filament single yarn of the cloth with the coating material resin (see claim 1 and FIG. 1, for example). This also discloses a coating method for obtaining such a coating film configuration, the method including using resin liquid, the viscosity of the coating material being within the range from 5000 to 20000 cP, and using a doctor knife with an acute-angled edge to set the contact pressure with the cloth 1 to 15 N/cm for coating (see claim 6, for example).
Patent Document 7 also discloses an airbag base fabric, in which a breathability reducing coating film of PA elastomer is fusion-formed on one or both of the sides of a cloth composed of polyamide (PA) fiber yarns (filaments) (see Abstract, for example). The coating material of the coating film is typically emulsion-based or organic solvent-based.
The organic solvent-based coating material is undesirable from an environmental viewpoint, and the coating facility is required to be of explosion-proof specification. For these reasons, the emulsion-based coating material, which is aqueous, is mainstream.
However, employing a conventional coating method (considered a knife coating) using an emulsion-based coating material would result in a coat formed on the surface of the cloth including the cloth interlace portions (seam portions) so that an air shutoff property (breathability reduction) is ensured as shown in FIG. 2 of Patent Document 5, but the coating material has a high viscosity and the resin is less likely to penetrate between single yarns and thereby enclose the single yarns, resulting in a reduction in the adhesion (adherence) between the cloth and the coat (see Paragraph [0015] of Patent Document 6). On the other hand, employing an immersion coating, in which the coating material has a low viscosity, would result in a penetration of the coating material in a manner enclosing almost all the single yarns as shown in FIG. 3 of Patent Document 5, but no resin film is formed on the cloth interlace portions and thus it is difficult to ensure a sufficient air shutoff property (see Paragraph [0015] of Patent Document 6).
It is therefore necessary, as in claim 6 of Patent Document 6, to force resin between single yarns so that the resin encloses each single yarn to ensure adhesion in a knife coating.
It should be noted that in the case of using resin (PA-based) of the same type as the cloth as in Patent Document 7, a fusion (baking) treatment after coating would provide adhesion as well as breathability reduction, but it is necessary to use a coating material based on special resin of the same type and having a specific melting point.
Moreover, the coating film thus forced and formed in the knife coating cannot have a uniform film thickness along the concavo-convex surface of the cloth as shown in FIG. 1 of Patent Document 5 (thickened at the cloth interlace portions). It is therefore conceivable that the amount of resin consumption might relatively increase.
Furthermore, in the case of Patent Document 7, since the coating film flows during fusion bonding, the resin flows into the interlace portions, so that the coating film is formed uniformly along the concavo-convex surface of the cloth as shown in FIG. 1 of Patent Document 7, while it is conceivable that actual coating films are formed with the interlace portions being slightly thickened.
It should be noted that the citation list of the present invention may also include Patent Document 8, though not influential to the patentability of the present invention. Patent Document 8 discloses an airbag base fabric, in which an aqueous emulsion coating material, which is a specific silicon-based processed composition, is added with a thickener to be adjusted to have a viscosity within a predetermined range and applied and hardened on a cloth to form a silicon-based rubber coat (breathability reducing coating film).